The simple answer is twofold

1. Their attack surface is massive. By getting all this data from millions of devices they are fantastic at detecting and finding solutions for zero day attacks. This is a big selling point for enterprise who care about zero day a whole lot

2. Free tier also used by IT tinkerers means when the question is asked “what X should we use? What have you used before?” CloudFlare is heads and shoulders above the rest. This is why Microsoft allowed a lot of world to pirate their OS.